1. Field of the Invention
The invention pertains to the field of color displays and more particularly to active and passive matrix liquid crystal displays.
2. Description of the Prior Art
Backlighted liquid crystal displays (LCD) with, for example, dimmable fluorescent backlight and twisted-nematic (TN) liquid crystals, have been developed to provide fiat panel displays for applications such as aircraft instrumentation, laptop and notebook computers, and the like. Such LCD's typically use liquid crystal material sandwiched between a back electrode, constructed as a matrix of transparent metal pixels or dot electrodes, and a front electrode made of a continuous transparent metal. The front electrode is often denoted as the common or counter electrode. Each pixel electrode is activated through a switch, usually implemented as a thin film transistor (TFT), which is deposited as a field effect transistor (FET). The drain electrode of each TFT is connected to, or actually forms, the pixel electrode with which it is associated. The gate electrodes of the TFTs in each row of the matrix are commonly connected to a gate bus-line for the row. The source electrodes of the TFTs in each column of the matrix are commonly connected to a source bus-line for the column. An image is created in raster fashion by sequentially scanning the gate bus rows while applying information signals to the source bus columns.
Such an arrangement may provide a monochrome display. Color capability for the LCD is established by positioning color filters at the front surface of respective pixels and grouping the pixels into color groups such as triads, quads, and the like, in, for example, diagonal or delta element arrays. For example, triads with primary color RED, GREEN and BLUE filters are often used. Various colors are generated at a pixel by appropriate video control of the gate and source electrodes of TFTs positioned at each filter cell. Video levels at the gate and source electrodes determine the polarization twist of the light as it traverses the liquid crystal associated with a primary color, thereby determining the intensity of the primary color light at each pixel that is transmitted through the polarizers. Such light intensity control of the primary colors generates the various colors of the display. Color liquid crystal displays, are usually manufactured with a uniform cell gap across the display active area and uniform twist angles across the liquid crystal for all color dots. Because of the properties of TN color mono-gap LCDs, a different level of off-state luminance occurs for each of the color dots. This phenomenon results in undesirably high levels of background luminance. The condition is exacerbated when the display is viewed from varying angles since each color dot changes luminance with viewing angle at different rates, some increasing and some decreasing. This aspect of mono-gap LCD technology results in high level of background luminance with viewing angle, producing undesirable secondary effects in viewability of display symbology. Additionally, objectionably different chromaticities of background color for various angles of view result.
Specifically, a RED, GREEN, BLUE (RGB) multicolor display requires an illumination source having strong spectral emissions at 435 nm, 545 nm, and 610 nm. It is impossible to obtain minimum background (off) transmission for all three wavelengths utilizing a display configured with a single cell gap with uniform twist angles. In such a mono-gap display, emissions from at least two of the three wavelengths leak through the display background resulting in increased background luminance. This, in turn, results in reduced contrast and a chromatic background.
It is desirable for a TN liquid crystal display (LCD) to exhibit a black background in the off state. As stated above, mono-gap LCDs with uniform twist angles exhibit undesirable high off state luminance, caused by light leakage through the liquid crystals. This leakage is primarily due to the wavelength dependence of the transmissions through the liquid crystals. A reduction in off state luminance is achieved with multi-gap construction, wherein the cell gap varies with the filter color. This type of construction, however, establishes three cell gaps, one for each color, the gaps being chosen for minimum transmission for the color transmitted through the cell. Though off state luminance is reduced by the multi-gap construction, the cell gap width must be carefully controlled if the performance gains are to be realized. The tight tolerances that must be maintained for the cell gaps significantly increase the cost of manufacture.